Field of the Invention
The invention relates to a method for the detection and location of changes in a component of a turbine, in particular in a turbine blade. It is directed, furthermore, to an apparatus for carrying out the method.
During operation of a turbine, for example a steam turbine, a measurable sound spectrum or acoustic spectrum (structure-borne noise) is emitted as a consequence of a reaction of individual components, in particular of the turbine blade, to different stresses. Such stresses are, for example, forces and temperature differences occurring during the expansion of a working agent. In addition, due to the energy conversion in the turbine, pressure fluctuations are produced which likewise lead to a characteristic acoustic emission of the components and are expressed in the so-called blade rotating noise. Irregularities in the operation of the turbine such as, for example, grazing of the runner blades of the turbine against the housing or against adjacent guide blades, or a loose turbine blade, also cause a disturbance-specific acoustic emission having a wide acoustic spectrum. The signal components in that acoustic spectrum differ with respect to the frequency and the intensity of the emitted acoustic energy and are characteristic for a turbine in normal operation, that is to say in fault-free operation, with the acoustic spectrum ranging from infrasound to ultrasound.
A method for determining and analyzing such acoustic emissions is disclosed in the publication entitled VGB Kraftwerkstechnik, Issue 9, September 1989, pages 899 to 907. Examples of a typical acoustic spectrum measured on the turbine housing and of an associated reference spectrum are represented in a simplified fashion in FIGS. 4 and 5, respectively. In the normal state of the turbine, characteristic signals, for example specific harmonic oscillations or frequencies f of the runner blade rotating noise having different intensities I emerge clearly, in the case of so-called structure-borne noise, from the reference spectrum represented in FIG. 5. In the case of the acoustic spectrum represented in FIG. 4, further signals are present in addition at different frequencies f having different intensities I. Those further signals are characteristic of specific changes, for example of the lack of turbine blades, or of a gear error in a gear disposed between the turbine and a generator.
In the known method, apart from the measurement of the structure-borne noise or blade rotating noise on the turbine housing, during operation of the turbine further measured values, for example the pressure and the temperature in the interior of the turbine, are determined and compared with standard values or desired values determined during fault-free operation of the turbine. In the event of a desired value deviation, a characterization of a coarse change which is present is possible by comparison with empirically determined faults or disturbances. However, early detection and exact location of a change which is only slowly proceeding in a specific component of the turbine are not possible, using that method. Consequently, to date damage has frequently not been detected until consequential damage had occurred and it had therefore been necessary to shut down the turbine at once. However, such an unscheduled shut down of a turbine, followed by an expensive fault-finding operation with the turbine housing open leads to down times which are extremely undesirable.